Many water-borne polyurethane dispersions chain-extended in various manners are known, which, by virtue of their excellent properties, are used inter alia as binders in water-based lacquers. Appropriately modified water-borne polyurethane dispersion are, however, also used as the hydroxy-functional component in isocyanate-crosslinking two-component clear or topcoat lacquers. However, isocyanate-crosslinking, two-component systems based on water-borne polyurethane dispersions in particular do not fulfil in all respects the requirements placed upon a multi-layer coating structure, in particular in automotive lacquer coating.
WO-A-94/03511 and WO-A-94/03512, for example, thus describe coating compositions based on polyisocyanates and water-borne OH-functional polyurethane resins. The polyurethane resins are obtained by separately producing an NCO-functional preadduct from organic acid having isocyanate-reactive groups and diisocyanates, together with an OH-functional pre-condensation product and then reacting the two-components. The disadvantages of these two-component systems are their unsatisfactory sag resistance and inadequate resistance to chemicals and petrol.
DE-A-44 13 562 describes aqueous polyurethane dispersions having a content of 2 to 150 mmol of siloxane linkages per 100 g of solid resin. The polyurethane dispersions are obtained by chain-extending a polyurethane prepolymer with siloxane groups by hydrolysis. The polyurethane prepolymer is obtained by reacting an NCO-functional polyurethane prepolymer with aminosilanes. The polyurethane dispersions are used in aqueous coating compositions as the sole binder or in combination with blocked polyisocyanates or formaldehyde condensation resins as the crosslinking agent. Two-component coating compositions based on the stated polyurethane dispersions and based on unblocked polyisocyanates require improvement with regard to sag resistance and blistering, especially in thick coats.